1. Field of the Invention
This invention relates generally to friction stir welding (FSW) and its variations including but not limited to friction stir processing (FSP), friction stir spot welding (FSSW), friction stir spot joining (FSSJ), friction bit joining (FBJ), friction stir fabrication (FSF) and friction stir mixing (FSM) (and hereinafter referred to collectively as “friction stir welding”).
2. Description of Related Art
Friction stir welding is a technology that has been developed for welding metals and metal alloys. Friction stir welding is generally a solid state process. Solid state processing is defined herein as a temporary transformation into a plasticized state that typically does not include a liquid phase. However, it is noted that some embodiments allow one or more elements to pass through a liquid phase, and still obtain the benefits of the present invention.
The friction stir welding process often involves engaging the material of two adjoining work pieces on either side of a joint by a rotating stir pin. Force is exerted to urge the pin and the work pieces together and frictional heating caused by the interaction between the pin, shoulder and the work pieces results in plasticization of the material on either side of the joint. The pin and shoulder combination or “FSW tip” is traversed along the joint, plasticizing material as it advances, and the plasticized material left in the wake of the advancing FSW tip cools to form a weld. The FSW tip can also be a tool without a pin so that the shoulder is processing another material through FSP.
FIG. 1 is a perspective view of a tool being used for friction stir welding that is characterized by a generally cylindrical tool 10 having a shank 8, a shoulder 12 and a pin 14 extending outward from the shoulder. The pin 14 is rotated against a work piece 16 until sufficient heat is generated, at which point the pin of the tool is plunged into the plasticized work piece material. Typically, the pin 14 is plunged into the work piece 16 until reaching the shoulder 12 which prevents further penetration into the work piece. The work piece 16 is often two sheets or plates of material that are butted together at a joint line 18. In this example, the pin 14 is plunged into the work piece 16 at the joint line 18.
Referring to FIG. 1, the frictional heat caused by rotational motion of the pin 14 against the work piece material 16 causes the work piece material to soften without reaching a meting point. The tool 10 is moved transversely along the joint line 18, thereby creating a weld as the plasticized material flows around the pin from a leading edge to a trailing edge along a tool path 20. The result is a solid phase bond at the joint line 18 along the tool path 20 that may be generally indistinguishable from the work piece material 16, in contrast to the welds produced when using conventional noon-FSW welding technologies.
It is observed that when the shoulder 12 contacts the surface of the work pieces, its rotation creates additional frictional heat that plasticizes a larger cylindrical column of material around the inserted pin 14. The shoulder 12 provides a forging force that contains the upward metal flow caused by the tool pin 14.
During friction stir welding, the area to be welded and the tool are moved relative to each other such that the tool traverses a desired length of the weld joint at a tool/work piece interface. The rotating friction stir welding tool 10 provides a continual hot working action, plasticizing metal within a narrow zone as it moves transversely along the base metal, while transporting metal from the leading edge of the pin 14 to its trailing edge. As the weld zone cools, there is typically no solidification as no liquid is created as the tool 10 passes. It is often the case, but not always, that the resulting weld is a defect-free, re-crystallized, fine grain microstructure formed in the area of the weld.
Travel speeds are typically 10 to 500 mm/min with rotation rates of 200 to 2000 rpm. Temperatures reached are usually close to, but below, solidus temperatures. Friction stir welding parameters are a function of a material's thermal properties, high temperature flow stress and penetration depth.
Previous patents have taught the benefits of being able to perform friction stir welding with materials that were previously considered to be functionally unweldable. Some of these materials are non-fusion weldable, or just difficult to weld at all. These materials include, for example, metal matrix composites, ferrous alloys such as steel and stainless to and non-ferrous materials. Another class of materials that were also able to take advantage of friction stir welding is the superalloys. Superalloys can be materials having a higher melting temperature bronze or aluminum, and may have other elements mixed in as well. Some examples of superalloys are nickel, iron-nickel, and cobalt-based alloys generally used at temperatures above 1000 degrees F. Additional elements commonly found in superalloys include, but are not limited to, chromium, molybdenum, tungsten, aluminum, titanium, niobium, tantalum, and rhenium.
It is noted that titanium is also a desirable material to use for friction stir welding. Titanium is a non-ferrous material, but has a higher melting point than other nonferrous materials. The previous patents teach that a tool for friction stir welding of high temperature materials is made of a material or materials that have a higher melting temperature than the material being friction stir welded. In some embodiments, a superabrasive was used in the tool, sometimes as a coating.
Friction Stir Welding (FSW) has been in use now for almost 20 years as a solid state joining process. This process has evolved from being used on aluminum or low melting temperature materials to high melting temperature materials such as steel, stainless steel, nickel base alloys and others. Literature is replete with tool geometries and process parameters needed to have a repeatable process. An understanding of the FSW process is important to understanding the invention described below.
While FIG. 1 describes the general joining process, there is one particular problem that was not described. Once a friction stir weld or friction stir processing pass is complete, the starting point of the joint may be left with material flash caused by the initial tool plunge. In many applications, having material flash disposed on a work piece after FSW is unacceptable.
One method for removing material flash is a run-on tab. However, using a run-on tab may also lead to a requirement for additional fixturing, work piece material, and post process removal methods. Furthermore, in many cases, a run-on tab is not an option because of space limitations, work piece geometry, cost, etc.
An example of an application where a run-on tab may not be an option would be using FSW to repair certain cracks. For example, nuclear reactor containment vessels may not have the option for a run-on tab. Furthermore, the material flash that may be left over from the FSW plunge may create a new corrosion crack initiation site and cannot be tolerated for safety reasons. There are many other examples of how a resulting FSW surface with material flash (hereinafter “flash”) may be detrimental to product performance, safety, and cost.
In some cases, flash resulting from the plunge is not the only detrimental effect resulting from FSW. Other problems from FSW may include unfavorable or detrimental residual stresses, tool undercut, flash along the weld due to tool wear or parameter selection, sharp flash locations creating safety concerns for human contact, inability to see sub-surface defects, fatigue life compromised by surface anomalies and others.
Having a consistent surface finish is preferred for engineered components in order to meet design and safety requirements. Thus, what is needed is a way to join Advanced High Strength Steels (AHSS) that can be used in the automotive and other industries.